Ion acceleration device



United States Patent 3,l55,858 lN ACQELERATKN DEVICE Edmund C. Lary, Vernon, Russell G. Meyerand, Jr.,

Glastonbury, and liranlr Salz, West Hartiord, Conn.,

assignors to United Aircraft Corporation, East Hartford, Conn., a corporation ot Belaware Filed May 28, 1962, Ser. No. 197,961 l2 Claims. {CL 313-63) This invention relates to an ion acceleration device. More particularly, it relates to an ion acceleration device especially suitable `for generating thrust for propulsion in s ace.

pSeveral approaches have been attempted for the electrical acceleration of a plasma for high specific impulse propulsion in space and for other applications. These include the electric arc jet, the cross-field magnetohydrodynamic accelerator, the c-esium-ion engine, and the Penning source accelerator. The electric arc jet is'critically limited by temperature-metallurgical considerations, and a Vfeasible coniiguration for space applications has not yet been found for the MHD device. The cesium-ion engine represents an attempt to accelerate ions separately from the electrons, and to have the two stream-s ultimately joined in the exhaust. In the cesium-ion engine the 1011 acceleration suffers from space-charge limitations, hence limited thrust, and from poor efficiency in the moderate speci-lic impulse regime which is important to interplanetary liight. The Penning source introduces the concept of the acceleration of ions through a space-charge neutralizing background of electrons which offers some promise of improved efliciency in the specilic impulse regime which is of interest in interplanetary lights.

In the present invention, ions are accelerated by an electric tield in the presence of space-change neutralizing electrons. Normally this is a very diliicult problem because the smaller mass of the electrons results in a much greater electron mobility in the electric field, and, hence, a great deal of power is wasted in the production of a high electron current. This difficulty is overcome in the present invention through the use of an annular acceleration chamber having an axial electric eld and a radial magnetic field so that the electric tield and the magnetic ield are substantially perpendicular to each other. The electric lield accelerates the ions and the electrons in opposite directions, and the magnetic lield is strong enough to cause the cycloidal motion of the electrons about ilux lines to dominate their motion while at the same time 1s small enough so that the ions are not significantly dellected. As a consequence, the electrons drift in a direction mutually orthogonal to the electric field E and the magnetic lield B, and the ions are accelerated through the drifting electrons without encountering space-charge limiting effects. y I

Accordingly, one feature of the present invention is a novel ion acceleration device in which ion-s are accelerated by an electric ieldin the presence of space-charge neutralizing electrons. Y

Another feature of the present invention is a novel ion acceleration device in which space-charge neutralizing electrons are caused to drift through the interaction of mutually perpendicular electric and magnetic iields while ions are accelerated through the neutralizing electrons by the electric ield.

Still another Ifeature of the present invention is a novel ion acceleration device in which an annular acceleration chamber has a radial magnetic field impressed thereacross and an axial electric field whereby ions are accelerated by the electric lield and electrons are caused to drift in the annular chamber to provide space-charge neutralization .for the ions.

Cice

Still another feature of the present invention is a novel ion acceleration device in which a radial magnetic lield and an axial electric iield interact in an annular acceleration chamber which will essentially produce a semipermeable membrane through which ions are accelerated in the direction of the electric field but through which axial movement of the electrons is inhibited by the magnetic iield.

Other features and advantages will be apparent from the specification and claims, Iand from the accompanying drawing which illustrates an embodiment of the invention, wherein the single figure is a sectional View of the ion acceleration device of the present invention.

Referring to the single i'igure, the ion acceleration device has a casing 2 with a cylindrical Wall 4l open at one end thereof. A centerbody 6 is centrally mounted within casing 2 and extends toward the open end of cylindrical wall 4. The centerbody 6 and the casing 2 cooperate to denne an annular chamber S having an open end, and both casing 2 and centerbody 6 are made of 'ferromagnetic material. The chamber 8 is lined with an electrically insulating shield 9.

A porous tungsten ring il@ is supplied with hot cesium gas through conduit l-Z to produce a supply or source of ions through contact ionization. Of course, it is to be understood that any other method of producing a supply of ions or a plasma could be used. Porous tungsten ring l@ is housed in and Asupported by a disc l mounted on centenbody 6, `and disc lf3 is maintained at a positive potential by power supply 14. A grid 16 is mounted at the end of centerbody 6 at least partially downstream of the open end of Wall 4, and grid lo constitutes a source -or supply of electrons, the electrons being generated by any well-known proces-s such as thermionic emission. Grid l5 is maintained at a negative potential by power supply 14. Grid 16 supplies electrons for neutralizing the ions upstream throughout .the acceleration chamber and also 4for neutralizing the ions after they have been discharged from the acceleration chamber. It is to be understood that the source of electrons is not restricted to grid 16 as shown but could be any type of electrode structure mounted, `for example, either on the centerbody 6 o1' the end of wall 4.

Magnetic lines of llux are generated by passing a current through coil l, and because of the ferromagnetic nature of centerbody 6 and casing 2 the lines of llux are drawn along centerbody 6 and across chamber 8 to wall 4 thus establishing a radial magnetic field across chamser 8 of substantially uniform strength along the length of the chamber. This magnetic field fringes across the ends of Wall 4 `and centerbody 6 so that it has an axial component at that location. It is desired to draw some of the electrons from grid lr6 into the chamber 8 to spacecharge neutralize the ions therein, and some of the electrons from grid lo are to be deposited in the space downstream of the discharge end of the accelerator to spacecharge neutralize the ions discharged from the accelerator. For this reason, at least part of the electron-producing grid 16 is positioned downstream of the discharge end of the accelerator so that some electrons will be constrained by the axial component of the magnetic field and remain outside of chamber 8. The positive potential on disc 13 and the negative potential on grid 16 establish an essentially axial electric field in chamber 8.

In the operation of the device, `a supply of ions is generated at plasma or ion source l0 and the ions are acy celerated downstream toward the discharge end of the clevice by the axial electric lield. Electrons emitted from grid 16 are drawn upstream by the electric iield arid hence move toward disc 13. As the electrons move upstream the combined etlect of the mutually perpendicular electric and magnetic lields in chamber 8 eventually causes the electrons to drift azimuthally around chamber 8 in a direction mutually perpendicular to the electric and magnetic fields while simultaneously spinning about their own guiding centers. This drift motion is illustrated schematically in the drawing byfthe tight.y swirled azimuthally directed arrow Vc. The ions, of course, have a much greater mass than the electrons, and the strength of the magnetic field is such that the ions are only slightly defiected as they pass through chamber 8 and are discharged through the open end of chamber 8. rThe drifting electrons inch'amber S are essentially immobilized axially for a period of time and hence each electron is available to space-charge neutralize a succession of ions in the beam of axially travelling ions. Collisions between the drifting electrons and the axially travelling ions will cause the drifting electrons to lose azimuthal momentum, and hence to diffuse upstream toward disc 13, the movement of the electrons upstream being such that the electrons will drift in a series of planes gradually approaching disc 13 until they are collected by positively charged disc 13.

Thus, a large ow of ions is space-charge neutralized within chamber d by a much smaller liow of electrons.

The ions are accelerated by the electric field and are discharged from chamber S to produce thrust. Electrons from grid 16 are also distributed .in the space downstream of the discharge end of chamber 8 to provide space-charge neutralization of the ions discharged from the acceleration device.

The annular configuration of chamber 8 prevents the build-up of an azimuthal electric field around the axis of the device, which electric field would impair the operation of the device. On the other hand, the insulating shield 9 allows the maintenance of the axial electric field required for ion acceleration and also allows a build up of a sheath of electrons on the side of shield 9 exposed to chamber 8 so that electron losses to the wall of chamber 8 are inhibited.

The primary operating parameters of the ion accelerator of this invention are: (l) the electron and ion gyrofrequeneies 01:623/111e and wizeB/mi, where e equals the magnitude of electronic charge, B equals the strength of the magnetic field, and me and mi are the mass of the electrons and ions, respectively; (2) the electron and ion collision frequencies uE and v1; (3) the desired ion beam speed /:glSp where g is the acceleration of gravity and 1'sp is the desired specific impulse in the ion engine; (4) the pressure in chamber 8 must be very low, on the order of a few millimeters of mercury or less-a condition which is inherently present in space. The condition that the electrons drift primarily in the azimuthal direction is that we be much greater than ue, and the condition that the ions not drift is that w1 be much less than V/L, where L is the length of the acceleration part of chamber 8.

Thus, it will be seen that we have invented and disclosed a new ion acceleration device, particularly suitable for use as an ion propulsion engine, in which only a very small electron current is needed to space-charge neutralize a large ion current. Thus, very little power is consumed in generating the electron current, and most of the input power can be utilized to accelerate the beam` of ions for the production of thrust. It can reasonably be expected that efficiencies of 80% or greater can be realized with our device.

It is to be understood that the invention is not limited to the specific embodiment herein illustrated and described but may be used in other ways without departure from its spirit as defined by the following claims.

We claim:

1. An ion acceleration device including a casing having an open end, a centerbody mounted within said casing and extending toward said open end, said centerbody and said casing forming a chamber therebetween, means for establishing a magnetic field across said chamber between said centerbody and said casing, a source of ions within said chamber, a source of electrons adjacent the open end of said chamber, and means for generating an electric field from within said chamber toward the open end of said casing perpendicular to said magnetic field.

2. An ion acceleration device as in claim 1 wherein said source of ions includes a porous tungsten element and a source of cesium gas in communication with said tungsten element.

3. An ion acceleration device as in claim 1 wherein said source of electrons includes a grid connected to said centerbody.

4. An ion acceleration device as in claim 1 including electric insulating means around said centerbody and around the inside of said casing.

5. An ion acceleration device including a casing having a substantially cylindrical wall and an open end, a substantially cylindrical centerbody within said casing substantially concentric with said cylindrical wall and extending toward said open end, said cylindrical wall and said centerbody forming an annular chamber therebetween, means for generating a radial magnetic field from said centerbody to said cylindrical wall, a source of ions within said chamber, a source of electrons adjacent the open end of said casing, and means for generating an electric field from within said chamber toward the open end of said casing perpendicular to said magnetic eld.

6. An ion acceleration device as in claim 5 wherein said source of ions includes a porous tungsten element and a source of cesium gas in communication with said tungsten element.

7. An ion acceleration device as in claim 5 wherein said source of electrons includes a grid connected to said centerbody.

8. An ion acceleration device as in claim 5 including an electric insulating means around said centerbody and around the inside of said casing.

9. An ion acceleration device as in claim 5 wherein said casing and said centerbody are both ferromagnetic.

10. An ion acceleration device including a cylindrical walled casing having an open end, a source of ions within said casing, a source of electrons adjacent the open end of said casing, means for establishing an electric field extending from within said casing toward said open end, whereby the ions are accelerated toward said open end and electrons are drawn within said casing, and means for generating a radial magnetic field from within said casing perpendicular to said electric field whereby the electrons are caused to drift and create an essentially neutral plasma within said casing.

11. An ion acceleration device as in claim l0 wherein said casing is ferromagnetic and wherein said means for generating the magnetic field includes a ferromagnetic centerbody mounted concentrically with said casing.

l2. An ion acceleration device including a cylindrical walled casing having an open end, a source of ions within said casing, a source of electrons adjacent the open end of said casing, and means including an electric field extending along the axis of said casing and a magnetic field extending radially from within said casing to the cylindrical wall for establishing a semi-permeable membrane to allow acceleration of the ions downstream toward the open end of said casing and inhibit the motion of electrons upstream in said casing.

No references cited. 

1. AN ION ACCELERATION DEVICE INCLUDING A CASING HAVING AN OPEN END, A CENTERBODY MOUNTED WITHIN SAID CASING AND EXTENDING TOWARD SAID OPEN END, SAID CENTERBODY AND SAID CASING FORMING A CHAMBER THEREBETWEEN, MEANS FOR ESTABLISHING A MAGNETIC FIELD ACROSS SAID CHAMBER BETWEEN SAID CENTERBODY AND SAID CASING, A SOURCE OF IONS WITHIN SAID CHAMBER, A SOURCE OF ELECTRONS ADJACENT THE OPEN END OF SAID CHAMBER, AND MEANS FOR GENERATING AN ELECTRIC FIELD FROM WITHIN SAID CHAMBER TOWARD THE OPEN END OF SAID CASING PERPENDICULAR TO SAID MAGNETIC FIELD. 